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Appendix V.                               975

and good feeds, such as may be obtained on newer and more
strongly-built machines. Attempts are being made, and with
some success, to remove all the surplus material at one cut, but
to do so very powerful tools are required, and in all cases * high-
speed steel,' or at least good Mushet or self-hardening steel, must
be adopted. As the whole question resolves itself into the cutting
of a certain amount of material in a given time, it is advisable to
compare one machine .with another regarding its capacity to
perform the work with rapidity; and to do this, the term ' power '
of a machine tool has been introduced, thus:

Power of) _ fdia." of largest \     (width" of \     (t velocity ratio I
a lathe   f    \ step of cone   J     1     step     J     \ of back gear j

The lathe in Plate V. would therefore have a
Power = 13 x 3i x f| x If = 518

and a very strong modern lathe would have a power of 750. To
obtain high results the diameter of the largest step should be
2 x centres, and the step should be wider than usual; the
bearings should be parallel, and the front one have a diameter =
\ x centres, while its length should be i| x diameter; and the
bed should have a total width = i|-x centres, being at the same
time deeper than formerly.

The speed must bear a definite relation to the cut and feed.
In a very powerful lathe, using high-speed cutting steel, the depth
of cut being ~J" to J", and the feed Ty to J-", the speeds may
be, for

Cast iron (with skin on)       ......    60 feet per min.

Mild steel (35 to 40 tons strength) ...    70            

Wrought iron............80            

Mild steel (25 to 30 tons strength) ...    90            

And with any good stiff lathe, using a good self-hardening tool


Gun metal (ordnance mixture)        ...  100 feet per min.
Gun metal (common)    "      ...        ...  120            


But the diameter of the work is also said to affect the cutting
speed. The figures given may be taken as true for a diameter